This invention relates to an improvement in pneumatic tire tread design. Specifically the improvement relates to an improved circumferential groove designed for tire treads. The invention applies to new tires and replacement treads employing such circumferential grooves. The design is particularly well suited for rib truck tires with circumferential grooves. The groove improvement provides limited protection to the base of such grooves from stone penetration and further reduces the occurrence of stone pickup and retention.
The rubber treads of pneumatic tires are commonly provided with one or more transversely spaced circumferentially continuous anti-skid grooves to increase the traction properties of the tire. These grooves are generally straight or zig-zag.
The spaced grooves define solid ribs, rib lugs or rib blocks of the road-contacting surface of the tire. The grooves extend radially inward from the ground contacting surface to a base, the base of the groove extending between the walls of adjacent ribs or blocks.
The grooves of such tires permit foreign objects such as stones to be picked up and retained. Occasionally a stone or other sharp object can be held or trapped by the sidewalls of the ribs. Such stones can be forced into contact with the base of the groove potentially cutting or penetrating the base of the groove. If the penetration is deep enough the wire reinforcements in the breaker or belt structure of the tire can be exposed. Such exposure can lead to oxidation or rusting of the metallic wire thus weakening the structure.
A second common problem is distinguishing between cuts in the base of a groove and surface cracks relating to ozone degradation. This is important because a cut in the thin rubber base of a groove can expose the reinforcing wire to oxidation. The oxidation weakens the wire and can result in premature failure due to wire breakage. Ozone surface cracks create surface blemishes that look similar to cuts but are not deep enough to expose the wire reinforcement.
Solutions to rocks penetrating a base of grooves have been attempted in the past.
A pneumatic tire with transverse crossbars or butresses is described in U.S. Pat. No. 3,706,334 granted Dec. 19, 1972.
A pneumatic tire with a rib occupying the majority of the base of a groove is described in French Patent 1,163,341.
Previous attempts to protect the base of the circumferential groove from rock penetration relied on rather large projections that occupied a high percentage of the groove void area. Such designs reduce the amount of water that the groove could accommodate resulting in a reduction of wet traction characteristics.
The invention described herein provides an effective stone penetration barrier while minimizing the amount of space used to provide such protection. Also the barrier is uniquely designed to give a visual indication of occurrences of rock penetration damage in the area of the base of the groove. The barrier design further enables one to distinguish surface ozone cracking from a deeper crack caused by rock penetration.